In recent years, there have been proposed organic field-effect transistors (hereinafter, referred to as an organic FET) using an organic semiconductor having excellent moldability as a semiconductor layer. Since a circuit pattern can be formed directly on a substrate through an ink-jet technique or screening technique by using the organic semiconductor as an ink, organic FETs using the organic semiconductor are actively studied in place of conventional field-effect transistors (hereinafter, referred to as an FET) using inorganic semiconductors. An important measure of FET performance is mobility. An improvement in the mobility means an increase in an ON-current. This means that a switching characteristic of the FET is improved, and for example, in liquid crystal display devices, this leads to realization of high gradation. For example, in the case of a liquid crystal display device, a mobility of 0.1 cm2/V·sec or more is required. Another important measure is hysteresis. The hysteresis represents a fluctuating range of a current value corresponding to a voltage hysteresis and the hysteresis value needs to be reduced for stable driving of the FET.
A gate insulating film is an important member of the FET. Generally, a semiconductor layer is formed in film form on a gate insulating layer composed of the gate insulating film, but since a channel, namely a current path is formed in the vicinity of an interface of the gate insulating layer in the semiconductor layer, properties of the gate insulating film (especially a surface thereof) have a large influence on FET characteristics. Common gate insulating films include inorganic films of silicon oxide and the like, but since expensive vacuum equipment such as CVD equipment is required in order to form inorganic films, the cost is increased. Further, this method has a problem that a high-temperature process is required for forming inorganic films and fabrication on a flexible substrate of plastic or the like is difficult. On the other hand, organic materials such as organic polymers have been actively studied because organic materials are soluble in an organic solvent and therefore a thin film can be formed at low cost by an application method such as ink-jet printing, and a gate insulating film can be prepared on the flexible substrate using a low-temperature process.
As an example of gate insulating materials of application type which are soluble in an organic solvent, a combination of polyvinylphenol and a crosslinking agent is known (for example, refer to Non-patent Document 1). However, since large amounts of polar groups such as an amino group and a phenol group remain in the materials, there is a problem of FET characteristics, particularly large hysteresis. Further, if the gate insulating material has such polar groups, in the case of patterning by a resist, the polar group can cause the permeation of chemicals into the insulating film or swelling in the steps of application, development and stripping of the resist to degrade the insulating film. Examples of other gate insulating films include a polysiloxane (for example, refer to Patent Documents 1 to 3), but a polysiloxane has a problem that, because of its high water repellency, the cissing of a coating liquid takes place in a process of applying a resist or an organic semiconductor onto a polysiloxane film and the formation of a uniform film is difficult. On the other hand, a method of improving coatability of a photoresist by a polysiloxane having an epoxy group introduced therein (for example, refer to Patent Document 4) is disclosed, but no example of an FET using the polysiloxane is reported.    Patent Document 1: Japanese Unexamined Patent Publication No. 2004-304121 (CLAIMS)    Patent Document 2: Japanese Unexamined Patent Publication No. 2005-120371 (CLAIMS)    Patent Document 3: Japanese Unexamined Patent Publication No. 2007-258663 (CLAIMS)    Patent Document 4: Japanese Unexamined Patent Publication No. 2007-119744 (CLAIMS)    Non-patent Document 1: “APPLIED PHYSICS LETTERS, vol. 82, p. 4175-4177 (2003)